1. Field of the Invention
The invention relates generally to a cathode for a field emission device, and more particularly to, a cathode for a field emission device capable of controlling the amount of electrons emitted from an emitter using a gate voltage with no regard to an anode voltage.
2. Description of the Prior Art
The field emission device is a core device constituting a cathode in a field emission display. The emission efficiency of the field emission device largely depends on the device structure, the emitter material and the shape of the emitter. Currently, the field emission device can be mainly classified into a diode type having a cathode electrode and an anode electrode, and a triode type having a cathode electrode, a gate electrode and an anode electrode depending on its structure. Materials forming the emitter may include metal, silicon, diamond, diamond-like carbon, carbon nanotube, and the like. Also, materials for forming the diode cathode consist of film or particles (or powder) and may include a diamond or carbon nanotube having a good electron emission characteristic in a low electric field. The diode cathode is disadvantageous in controllability of electron emission and low-voltage driving but is advantageous in the manufacturing process and reliability of electron emission compared to the triode type.
FIG. 1 is a cross-sectional view of a device for explaining a field emission device using a conventional cathode.
Referring now to FIG. 1, a cathode 100 includes a dielectric layer 140 and a gate electrode 150 sequentially formed on a lower substrate 110. A gate hole 170 is formed in a given region of the dielectric layer 140 and the gate electrode 150. Also, a catalytic layer 130 is formed on the lower substrate 110, exposed through the gate hole 170 and the emitter 180 is also formed on the catalytic layer 130.
Meanwhile, an anode plate 195 is located at a position facing the cathode 100 by a given distance. The anode plate 195 has an upper substrate 196 in which an anode electrode 197 and a fluorescent material 198 are stacked.
In the above, the cathode electrode is included in the lower substrate 110. Materials of the lower substrate 110 may include a glass substrate, a silicon wafer, a dielectric substance on which a conductive material is coated, etc. The dielectric layer 140 may be formed by electron beam evaporator or plasma enhanced chemical vapor deposition (PECVD) method. The gate electrode 150 is made of a metal film and may be formed by sputtering or electron beam deposition method. The gate hole 170 may be formed by photolithography process and reactive ion etching (RIE) process. The catalytic layer 130 is formed of a transition metal series. For example, the catalytic layer 130 may be formed of Ni, Fe or Co. The catalytic layer 130 may be formed by sputtering or electron beam deposition method as like in the method of forming the gate electrode 150. The emitter 180 is made of any one of carbon nanotube, carbon nanoparticles and carbon fiber and may be formed by plasma chemical deposition method or thermal chemical vapor deposition method.
An operation of the cathode 100 formed by the above method will be described as follows.
If a voltage (Va) applied to the anode electrode 197 is consecutively increased, electrons are emitted from the emitter 180 even though a voltage (Vg) is not applied to the gate electrode 150. The emitted electrons cause a fluorescent phenomenon while colliding with the fluorescent material 198.
Meanwhile, if the voltage (Vg) is applied to the gate electrode 150, the fluorescent phenomenon can be controlled by a small amount of the voltage (Vg) as the distance between the emitter 180 and the gate electrode 150 is smaller than that between the emitter 180 and the anode electrode 197.
The cathode 100 has a triode cathode structure. Therefore, there is an advantage that the cathode 100 can be controlled by a very small operating voltage compared to the diode cathode. However, in order to obtain a screen having a high brightness, it is required that the gate voltage (Vg) and the anode voltage (Va) be increased simultaneously. Further, in order to obtain a further higher brightness, it is required that the anode voltage (Va) be further increased. In this case, electrons, which are emitted from an edge of the emitter 180 near the gate electrode 150, are controlled by the gate voltage (Vg). However, electrons, which are emitted from a central portion of the emitter 180 relatively far spaced from the gate electrode 150, cannot be controlled by the gate voltage (Vg). Therefore, electrons are only emitted by the anode voltage (Va).
If the anode voltage (Va) is increased in the conventional triode cathode, a high brightness can be obtained but a dark state of the screen could not be implemented, as described above. Therefore, the contrast characteristic of the screen is degraded.
The conventional triode cathode is complicated in structure compared to the diode cathode, but could have a decreased operating voltage. However, if the anode voltage (Va) is increased even when the gate voltage (Vg) is not applied, there is a problem that the amount of the electrons emitted from the emitter 180 could not be controlled by the gate voltage (Vg) since electrons are emitted from the emitter 180.
The present invention is contrived to solve the above problems and an object of the present invention is to provide a cathode for a field emission device capable of improving the contrast characteristic even at a lower anode voltage and easily controlling electrons emitted from an emitter by a gate voltage, by forming a catalytic layer at the side of a gate hole and growing the emitter in the catalytic layer to distribute a electric field generated by a voltage applied to a gate electrode over all the portions of the emitter.
In order to accomplish the above object, a cathode for use in a field emission device comprising a catalytic layer and a gate electrode formed in a stack structure along with a dielectric layer on a substrate, an emitter, a gate hole exposing the substrate according to the present invention, is characterized in that the emitter is located at the sidewall of the catalytic layer exposed through the gate hole.
In the above, the gate electrode and the catalytic layer are located at opposing sides centering around the gate hole and are located at different heights. The gate electrode located to the substrate nearer than the emitter.
A cathode for use in a field emission device according to the present invention, is characterized in that it a dielectric layer and a catalytic layer stacked on a substrate; a gate hole exposing the substrate; a gate electrode formed at a given region of the exposed substrate; and an emitter formed at the sidewall of the catalytic layer exposed through the gate hole.
The emitter is made of any one of carbon nanotube, carbon nano particles and diamond having defects using carbon as a major component. The catalytic layer is made of one of transition metals such as Fe, Co and Ni or an alloy or a compound of the transition metals. The catalytic layer is used as a cathode electrode. A cathode electrode is further formed between the catalytic layer and the dielectric layer.